Satellite Signal Acquisition, Tracking, and Data Demodulation - Understanding GPS: Principles and Applications - page 233

1
2

( )

10 12 14 16

P ≤ 36 D + 211 D + 1404 D + 11633 D

b

(5.57)

1 C

N

D = exp −

2 R

b 0

It should be noted that both of these BER expressions presume that the PLL is
perfectly tracking carrier phase without any slips. At low C/N 0 's, phase tracking
errors degrade the BER. If the C/N 0 is too low or if the signal dynamics are too
severe, then as discussed in Section 5.6.1 the PLL is unable to track carrier phase,
and data demodulation can no longer be performed.
Before detection of the data bits can commence, bit synchronization must be
performed. This function may be performed by accumulating I PS outputs over all
possible start/end points and then comparing the power seen in each possibility. For
instance, with the C/A code and P(Y) code signals, the I PS outputs are normally
based upon 1-ms integrations. There are 20 I PS outputs per data bit and 20 possible
bit edge timing possibilities to explore. It should be noted that if the receiver makes
an error of 1 ms or more in estimating the location of a data bit edge, the results
could be catastrophic. As noted in Section 5.7.1.5, a 300-km pseudorange error
would be seen for a 1-ms error. Fortunately, robust detection of the correct bit edge
location is not difficult via the previously mentioned technique if the power in the
20-ms accumulated I PS outputs (i.e., the square of the 20-ms accumulations) for each
of the 20 bit edge possibilities are compared over many bit durations.

5.11 Special Baseband Functions

Numerous special baseband functions must be implemented in a GPS receiver
design, but the following three design examples are among the most important.

5.11.1 Signal-to-Noise Power Ratio Meter
An accurate measure of C/N 0 in each receiver tracking channel is probably the most
important mode and quality control parameter in the receiver baseband area. The
basic C/N 0 meter design in Figure 5.42 shows that prompt I and Q signals and noise
samples from the same channel are integrated and dumped using K samples (typi-
cally over the maximum 20-ms predetection integration time). A power envelope is
formed from the averaged prompt I and Q signals. This is passed through a lowpass
filter (LPF) and output as an estimate of the carrier power for the numerator (even
though it also contains noise). The averaged quadraphase components of the noise
samples is scaled, then squared and passed through a LPF to form an estimate of the
noise power for the denominator. The C/N 0 (ratio-Hz) estimate is formed by divid-
ing the numerator ( C ) by the denominator ( N 0 ).

5.11.2 Phase Lock Detector with Optimistic and Pessimistic Decisions
Many receiver control decisions are made based on the phase lock detector. Some
applications require more certainty of phase lock than others. Figure 5.43 illustrates